This renewal proposal is designed to investigate the mechanisms of hyperbaric oxygen induced preconditioning (HBO-PC) in a focal cerebral ischemic model produced by Middle Cerebral Artery Occlusion (MCAO). For broadening of clinical applicability, it was also designed to evaluate efficacy of HBO-PC against other brain injuries by using a global cerebral ischemia model produced by Four Vessel Occlusion (4VO); and a Surgical Brain Injury (SBI) model which represents brain injury caused by neurosurgical procedures. Based on these preliminary results demonstrating HIF-1 specifically responds to HBO, publications by us and others, and the fact that HIF-1 is the main oxygen sensing protein, our Central Hypothesis is that HBO produces preconditioning by transiently inducing HIF-1 and its downstream target genes responsible for cell death and blood-brain barrier (BBB) disruption and thereby leads to ischemic tolerance to subsequent major brain insult. The following specific aims are proposed to address our hypothesis. Aim 1: To determine the mechanism of HIF-1 induction by HBO-PC in brain tissue and broad efficacy of HBO-PC. Our specific hypothesis is that HBO-PC produces moderate levels of reactive oxygen species (ROS) which, by destabilization of prolyl hydroxylase 2 (PHD2), induces HIF-1 accumulation in brain tissue. Interventions such as quenching ROS or enhancing PHD2 will abolish induction of HIF-1 by HBO-PC and thereby abolish preconditioning effects. It is expected that HBO-PC is a broadly effective modality that reduces brain injuries as a consequence of MCAO, SBI, and 4VO models. Aim 2: To determine the mechanism by which HIF-1 induction leads to tolerance against apoptotic cell death after cerebral ischemia. Our specific hypothesis is that HIF-1, induced by HBO-PC, binds to and stabilizes p53. This p53 accumulation and activation of its downstream proteins: Bax and caspase-3 result in tolerance against subsequent cerebral ischemia-induced cell death. Interventions such as pharmacological inhibition of p53 and caspase-3 are expected to abolish HBO-PC-induced cell protection. Aim 3: To determine the mechanism by which HIF-1 induction leads to tolerance against BBB disruption after cerebral ischemia. Our specific hypothesis is that HIF-1, induced by HBO-PC, activates VEGF and its down streams through three possible pathways which will provide tolerance against subsequent cerebral ischemia-induced BBB disruption and brain edema. The long term goal of this proposal is to establish HBO-PC as a clinically means to prevent or reduce brain injuries that can either complicate major cardiac or liver surgeries (4VO model) or that are a result of neurosurgical procedures (MCAO, SBI models). To pursue this goal we will establish the mechanistic basis for HBO-PC at a molecular level. We will also determine the long term beneficial effect of HBO-PC as measured by neurological and neurobehavioral outcomes. The clinical implication of HBO-PC, as an effective, easy to use, and safe modality, could be expanded to multiple brain injuries complicating major surgical procedures.